1. Field
The following description generally relates to a Light Emitting Diode (LED) driving apparatus, and such as, for example, an LED driving apparatus that is capable of detecting the degradation of an LED array.
2. Description of Related Art
Liquid Crystal Displays (LCDs) are widely used because they are relatively thin and light-weight in comparison to many other display apparatuses. In addition, LCDs require a low driving voltage and electricity to operate in comparison to many other display apparatuses. However, because a liquid crystal panel by itself does not emit light, an LCD requires a separate backlight unit (BLU) to supply light to the liquid crystal panel.
A Cold Cathode Fluorescent Lamp (CCFL) or a Light Emitting Diode (LED) is often used as a light source for the backlight unit of an LCD. However, CCFLs use mercury in their fluorescent lamps; thus, CCFLs impose risks of environmental pollution. Further, CCFLs have shortcomings such as slow responsiveness, low color representation, and the like. In addition, CCFLs are not suitable for an LCD that is light-weight, thin, short and/or small in size.
On the contrary, LEDs are environment-friendly in that LEDs do not use environmentally harmful materials such as mercury, and enable impulse driving. In addition, an LED backlight unit is capable of providing a good color representation because the brightness or color temperature of the LED backlight unit can be freely controlled by appropriately adjusting the luminescence of red, green, and blue light emitting diodes. Further, LEDs are also suitable for obtaining an LCD that is light, thin, short and/or small. Thus, LEDs have recently been widely used as the backlight source for LCDs, or the like.
When a plurality of LEDs are connected in series to each other in an LCD backlight that utilizes LEDs, such as when an LED array is used as the backlight source for an LCD panel, a driving circuit may be necessary to provide a constant current to the LEDs, and a DC-DC converter may be necessary to regulate electricity supplied to the LEDs.
Meanwhile, considering that an LED array sometimes fails ‘open’ or ‘short’ due to a prolonged driving of the array or due to a physical impact, a protective circuit may be necessary to detect the degradation of an LED array. For instance, an individual LED in an LED array may fail electrically ‘open’ or electrically ‘short.’ When an individual LED fails ‘open,’ the electric circuit is in an open state and the power supply may be cut off from other LEDs connected in series with the failed LED. In instances in which an individual LED fails ‘short,’ the current continues to flow through other LEDs connected in series with the failed LED.
For example, a protective circuit may detect the degradation of the LED array by measuring the forward voltage (VFB) of an LED array. However, a conventional protective circuit may erroneously determine that an LED array has degraded when actually the settling time of the constant current source is slow or when an abnormal forward voltage (VFB) is detected due to a peak current of the constant current, irrespective of whether the LED array has actually degraded.
Referring to FIG. 6, the current flows to the LED array as a dimming signal (PWM signal) is on, causing the forward voltage (VFB) to gradually decrease. However, due to a rather slow settling time of the constant current source, if the forward voltage drops as illustrated by VFB[A] in FIG. 6, the conventional protective circuit may measure the forward voltage in such a section. When the forward voltage is measured in such a section, the measured forward voltage is higher than a normal one, and as a result, the protective circuit may determine that the corresponding LED array has failed short.
Also, the conventional protective circuit sometimes measures forward voltage in the section when the forward voltage drops close to 0V due to the peak current of the constant current. However, when the forward voltage is measured in such a section, the measured voltage is lower than a normal forward voltage; as a result, the protective circuit may determine that the LED array has failed open.